This invention relates to communications systems, and, more particularly, to antennas and antenna reflectors used therein.
Satellites are used to carry a large part of the private and government communications throughout the United States and the world. In the most basic satellite communications system, a ground sending antenna transmits a signal to a satellite receiving antenna mounted on a satellite in orbit above the earth. The signal is then transmitted by a transmitting antenna on the satellite to other spacecraft or to a ground receiving antenna. The signal can also be modified by the satellite, as by amplifying it before retransmission. Because the satellite is usually thousands of miles above the earth in a geostationary orbit, this procedure allows signals to be transmitted through the satellite to ground receiving stations thousands of miles from the ground sending station. This satellite transmission technique makes possible the beaming of television signals from central locations directly to local cable companies and backyard television dish receivers, as well as voice, data and other types of transmissions.
The two antennas located on the spacecraft are important components of this system, since all signals pass through the antennas on their way to and from the satellite. Each antenna typically includes an antenna reflector in the shape of a parabola and a transceiver at the focus of the parabola. A parabolic antenna has the feature that the energy of a wide signal beam received by the antenna reflector is directed to the focus of the parabola for receipt by the transceiver, or that energy radiated by a transceiver at the focus is directed in a parallel beam toward the ground by the antenna reflector. The antenna reflector is the part of the antenna that is most visible and recognizable, since the antenna reflector design is somewhat similar to the large dish seen in ground-based antennas.
The material used to construct the antenna reflector should have a number of characteristics, which heretofore have not been attained in antenna reflector materials. The material should be formable into a parabolic reflector with sufficiently high modulus of elasticity and strength to hold its shape precisely during launch, deployment and use. Distortions to the antenna reflector can cause significant degradation in the antenna performance and hence the quality of signal received by the ground station. The material should also have a low coefficient of thermal expansion and expand generally isotropically, to minimize the effects of heating and cooling as the antenna reflector passes from direct sunlight to shade and back into direct sunlight. When the antenna reflector is heated by the sun, it expands. The expansion ideally would be negligibly small, so that uneven heating would not cause the antenna reflector to warp, again resulting in degradation of signal. Even if the antenna reflector is heated evenly, anisotropic expansion can cause distortion. It is therefore desirable that the material of which the antenna reflector is constructed expand very little upon heating, and that such expansion be uniform.
The material of the antenna reflector must also meet the electrical design parameters required of all antenna reflectors. It must have a surface finish that does not interfere with the signal. Since the signal in some cases is a microwave signal with a very short wavelength of about one-fifth of a thousandth of an inch, the surface of the antenna must be even smoother than this to avoid interference with the signal. The material should have a low dielectric constant to minimize refraction of the signal by the antenna reflector, and a low dissipation factor to minimize attenuation or signal loss by the reflector itself. The material must allow a thin metallic coating to be applied to the surface of the antenna reflector to reflect the signal.
The antenna reflector must also meet new and evolving requirements for components used on satellites. The cost of boosting weight to orbit is high, so that the antenna reflector should be light and formed of a material of low density. The material should experience low outgassing in a space environment, since evolution of gas can interfere with operation of the satellite. It is also highly desirable that the material be stable to high temperatures to resist damage by intense beams of energy that might be directed against the antenna reflector, such as a high intensity laser beam that would burn holes in the antenna reflector, or even distort it substantially. High temperature capability would allow the antenna reflector to resist attack by lasers or other directed energy weapons without the need for specialized shielding and defense measures. Finally, it would be desirable to construct the antenna reflector of a material having characteristics such as density, surface structure, and composition that can be varied over ranges of values to permit antenna designers flexibility in their selection of antenna reflectors for different requirements, always using a basic material of construction with which they are familar and for which data is readily available.
This combination of characteristics has not heretofore been available. Existing antenna reflectors have been constructed of metals, composite materials or honeycomb structure, but these materials of construction do not have the combination of properties described above. Metals are heavy, have high coefficients of thermal expansion, cannot be readily tailored to different missions, and, in many cases, are susceptible to laser damage. Composites may have anisotropic coefficients of thermal expansion, may not be electrically acceptable, and are usually very suceptible to laser damage. Honeycomb materials have highly anisotropic expansion, may have unacceptably low mechanical properties, often cannot be made without surface porosity that can interfere with signals of short wavelength, and cannot be readily tailored to special requirements.
Accordingly, there is an ongoing need for an improved material for antenna reflectors, particularly for microwave spacecraft and satellite antenna reflectors for microwave signals. The present invention fulfills this need, and further provides related advantages.